Elongation of wood fibers combines features of diffuse and tip growth

Hemicelluloses are polysaccharides in plant cell walls that have beta-(1-->4)-linked backbones with an equatorial configuration. Hemicelluloses include xyloglucans, xylans, mannans and glucomannans, and beta-(1-->3,1-->4)-glucans. These types of hemicelluloses are present in the cell walls of all terrestrial plants, except for beta-(1-->3,1-->4)-glucans, which are restricted to Poales and a few other groups. The detailed structure of the hemicelluloses and their abundance vary widely between different species and cell types. The most important biological role of hemicelluloses is their contribution to strengthening the cell wall by interaction with cellulose and, in some walls, with lignin. These features are discussed in relation to widely accepted models of the primary wall. Hemicelluloses are synthesized by glycosyltransferases located in the Golgi membranes. Many glycosyltransferases needed for biosynthesis of xyloglucans and mannans are known. In contrast, the biosynthesis of xylans and beta-(1-->3,1-->4)-glucans remains very elusive, and recent studies have led to more questions than answers.

Tissue mechanics have been shown to play a key role in the regulation of morphogenesis in animals [1-4] and may have an equally important role in plants [5-9]. The aerial organs of plants are formed at the shoot apical meristem following a specific phyllotactic pattern [10]. The initiation of an organ from the meristem requires a highly localized irreversible surface deformation, which depends on the demethylesterification of cell wall pectins [11]. Here, we used atomic force microscopy (AFM) to investigate whether these chemical changes lead to changes in tissue mechanics. By mapping the viscoelasticity and elasticity in living meristems, we observed increases in tissue elasticity, correlated with pectin demethylesterification, in primordia and at the site of incipient organs. Measurements of tissue elasticity at various depths showed that, at the site of incipient primordia, the first increases occurred in subepidermal tissues. The results support the following causal sequence of events: (1) demethylesterification of pectin is triggered in subepidermal tissue layers, (2) this contributes to an increase in elasticity of these layers-the first observable mechanical event in organ initiation, and (3) the process propagates to the epidermis during the outgrowth of the organ. Copyright © 2011 Elsevier Ltd. All rights reserved.

A collection of 130 new plant cell wall glycan-directed monoclonal antibodies (mAbs) was generated with the aim of facilitating in-depth analysis of cell wall glycans. An enzyme-linked immunosorbent assay-based screen against a diverse panel of 54 plant polysaccharides was used to characterize the binding patterns of these new mAbs, together with 50 other previously generated mAbs, against plant cell wall glycans. Hierarchical clustering analysis was used to group these mAbs based on the polysaccharide recognition patterns observed. The mAb groupings in the resulting cladogram were further verified by immunolocalization studies in Arabidopsis (Arabidopsis thaliana) stems. The mAbs could be resolved into 19 clades of antibodies that recognize distinct epitopes present on all major classes of plant cell wall glycans, including arabinogalactans (both protein- and polysaccharide-linked), pectins (homogalacturonan, rhamnogalacturonan I), xyloglucans, xylans, mannans, and glucans. In most cases, multiple subclades of antibodies were observed to bind to each glycan class, suggesting that the mAbs in these subgroups recognize distinct epitopes present on the cell wall glycans. The epitopes recognized by many of the mAbs in the toolkit, particularly those recognizing arabinose- and/or galactose-containing structures, are present on more than one glycan class, consistent with the known structural diversity and complexity of plant cell wall glycans. Thus, these cell wall glycan-directed mAbs should be viewed and utilized as epitope-specific, rather than polymer-specific, probes. The current world-wide toolkit of approximately 180 glycan-directed antibodies from various laboratories provides a large and diverse set of probes for studies of plant cell wall structure, function, dynamics, and biosynthesis.